The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art. These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies, and the definition of that term in the reference does not apply.
Coal gasification is the process of converting coal into a fuel gas mixture commonly referred to as “syngas” or “synthesis gas”. Syngas primarily consists of hydrogen, methane (CH4), carbon monoxide (CO), and water vapor, and is highly combustible. Syngas can be burned to generate heat for various applications, including cooking, illumination, and electricity generation. Large-scale coal gasification is primarily used for the latter and is responsible for supplying a significant portion of our electricity consumption.
Unfortunately, coal gasification has many harmful by-products that can negatively impact the environment. One of those by-products is “sour water” (also “grey water” or “waste water”) which is produced when water is used to quench, cool, and/or clean syngas. Sour water can contain high amounts of ammonia (NH3), hydrogen sulfide (H2S), thiosulfate, cyanides, chlorides, fluorides, carbon dioxide, and carbonates. Sour water may also contain suspended solids and minerals such as calcium, magnesium, and silica. In addition, sour water can contain significant levels of tar and organics such as mixed phenols. The exact composition of sour water will depend on coal composition, type of gasifier (e.g., temperature/pressure, slagging/non-slagging, wet/dry type particulate removal, etc.).
Various methods for treating and/or disposing of sour water are known. For example, CN101597092, U.S. Pat. Nos. 3,804,757, 4,253,928 describe several methods of treating coal gasification wastewater using a steam stripping tower. As another example, U.S. Pat. No. 4,239,620 discusses methods of removing cyanide from sour water. As yet another example, U.S. Pat. No. 4,746,434 describes a method of biologically treating sour water prior to disposal.
One of the problems associated with sour water treatment is its tendency to corrode process devices (e.g., stripper towers, conduits, etc.), cause scaling, and exert toxicity on downstream biological treatment section(s) or in the environmental receptors of the treated sour water. U.S. Pat. No. 4,505,881 describes converting cyanide into ammonium thiocyanate prior to routing the sour water to a stripper for H2S and NH3 removal. This approach helps somewhat to address corrosion in the stripper; however, it requires a post-hydrolysis process after the stripper to remove ammonium thiocyanate. This is disadvantageous because the temperature required for hydrolysis uses energy. Similar approaches are described in U.S. Pat. Nos. 4,737,289 and 5,431,877. Unfortunately, these methods suffer from similar drawbacks.
Current processes and systems for handling sour water in an environmental fashion are not well evolved. A growing awareness of our environmental footprint has led to the need and desire for better methods of treating sour water.
Thus, there is still a need for improved methods and systems for treating sour water produced from industrial processes such as coal gasification.